All the textures that your models use are images, (normally in JPG format) , so, ..yes, is the simple answer.
It depends on what the photo is and what you’re trying to do.
but, simply put,.. you can change the image used on a model, for another image, but, depending on the image you use it may not look as you expect it to.
you cannot just take a photo of someone, and add that straight onto the model, and it will all work perfectly.
Models are divided into “shading domains”, and each shading domain has a different “UV mapping” which tells the 3D program how to “project” that 2D image, onto the surface of the 3D Model.
Take a look at some of the Skin textures you have,. in any image viewer, and you’ll see that it’s a flattened out (unwrapped) skin image, which is made using several different images combined with a lot of care and skill.
you’ll also see several different images used ion a single model to texture different parts (shading domains).
in Daz Studio,. you can go to the “Surfaces” tab, and select the edit option, then you’ll see a list of all the shading domains in the model, and you can select one of these shading domains and see the attributes of the shader, such as the colour or Images used.
and you can replace any of these images with another one.
But, as I said, you may not get what you expected if you simply load a photo and apply it to a model.
As a “bad” example… here’s a landscape image applied to the Genesis figures face.
Hope some of that makes some sense